Hypertension is treated by maintaining a patient on an antihypertensive drug for a long time and, therefore, antihypertensive drugs are demanded not only to exhibit potent antihypertensive activity but to cause no undesirable side effects throughout the long-term administration. That is, antihypertensive drugs are required (1) to have potent and long-lasting antihypertensive activity; (2) to show slow onset of antihypertensive action because quick onset of the action will significantly change the homeostatis of circulatory organs (see Life Science, Vol. 47, pp. 1693-1705 (1990)), readily causing reflex tachycardia (see Japanese Journal of Pharmacology, Vol. 58 (Suppl. 1), p. 36 (1992)); and desirably (3) to increase the renal blood flow because a blood flow disorder in the kidney closely relates to the maintenance and progress of hypertension and an increase in renal blood flow is expected to alleviate hypertension (see Circulation, Vol. 69, pp. 1142-1145 (1984)).
A compound represented by formula (IV): ##STR2## wherein R represents a hydrogen atom, an alkyl group, an aryl group, an aralkyl group, etc., is known to exhibit potent and long-lasting antihypertensive activity attributed to potassium channel activation and therefore be useful as an antihypertensive drug (see JP-A-2-145584, JP-A-3-20275, and Journal of Medicinal Chemistry, Vol. 33, pp. 2759-2767 (1990); the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Although the compounds of formula (IV) almost satisfy the requirements for potency and duration of antihypertensive activity, they are still unsatisfactory in terms of the pattern of onset of antihypertensive action and renal blood flow increasing activity.